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Attitude Determination and Control System (ADCS) perfromance is critical for Microsatelltes and Nanosatellites missions

Nanosatellite and microsatellite refer to miniaturized satellites in terms of size and weight, in the range of 1-10 Kg and 10-100 kg, respectively. These are the fastest growing segments in the satellite industry. ‘CubeSat’ is one of the most popular types of miniaturized satellites. CubeSats were made possible by the ongoing miniaturization of electronics, which allows instruments such as cameras to ride into orbit at a fraction of the size of what was required at the beginning of the space age in the 1960s. The missions of micro-satellites range from early military usages to weather forecast, resources exploration, communication, and scientific experiment.

 

The trend toward small-sized spacecraft continues in government applications and is even increasing in commercial space endeavors that are funded by venture capital. Small spacecraft, including nanosatellites, microsatellites, and small satellites (smallsats), are an attractive alternative to traditional, larger spacecraft due to reduced development costs, decreased launch costs, and increased launch opportunities. CubeSats reduce launch costs in two fundamental ways. They don’t weigh that much, which means a rocket doesn’t need a lot of fuel to heft them. In most cases, they also share a rocket with a larger satellite, making it possible to get to space on the coattails of the heavier payload.

 

One of the major advantages of nano and microsatellites are reduction in delay and low cost of building and operating these satellites. This means strongly reducing spacecraft lifecycle costs and lead time, without reducing (and most likely increasing) performance. In turn, this would allow the full potential of space to be exploited and space-based systems to be competitive with ground-based systems that provide similar services.

 

Modern small satellites has not only small size, light weight, high technology, good performance, high reliability, short development cycle, but also adaptability, ease of management, low risk, and thus has a broad development and application prospects. It can be used as a single satellite, but also satellites constellation.

 

Attitude control are the actions carried out to control the orientation of an object in relation with a specified reference system. The term is often used in the aerospace sector because it is used with regard to the orientation of Spacecraft and satellites in relation with the firmament, certain regions or nearby objects. In a nutshell, it is the compass that is used in outer space.

 

Attitude Determination and Control System deals with the position and orientation of the satellite in space, which is required for maintaining stability and maneuvering for imaging and communications. The attitude control system architecture is a crucial subsystem for any satellite mission since precise pointing is often required to meet mission objectives. The accuracy and precision requirements are even more challenging for small satellites where limited volume, mass, and power are available for the attitude control system hardware.

 

 

Control System

The effectiveness of microsatellites’ application and, therefore, the effectiveness of the practical tasks, solved by them, significantly depends on technical characteristics of a control system (CS) of a satellite, therefore requirements to accuracy and operating performances of CS are strict enough. At the same time constraints on weight and power consumption cause the minimum of devices’ structure, it complicates algorithmic support of CS very much. In this connection there is a task of creation of a control system for the microsatellite with devices’ structure which ensuring minimum energy-mass and cost performances.

 

The objectives of the Control System are microsatellite angular motion control; onboard equipment control; computation of current orientation parameters; and -computation of current navigation parameters. Therefore the control system includes the following subsystems: orientation and stabilization control subsystem; attitude determination subsystem; autonomous magnetometric navigation subsystem; and on-board equipment control subsystem.

 

The major functional requirements of AOCS:

i. Stabilization of the satellite after separation;
ii. Automatically attitude acquisition and attitude determination;
iii. Stabilization pointing and control of the attitude and angular rate in 3 axes with very severe High
Stabilization Pointing requirements;
iv. Attitude manoeuvres to allow for pointing the sky according to the guidance law;
v. Orbit maintenance and orbital debris avoidance manoeuvres during the whole lifetime.

 

The AOCS functional architecture is based on the classical set of functions to implement the attitude control loop: attitude sensor processing, attitude estimation (sensor data fusion), attitude guidance (attitude profile commanded by the ground), control laws and actuator command
processing. The AOCS also includes additional functions: ground interface (telecommand and telemetry), sensor commanding (for instance, FGS management), and thruster management for orbit manoeuvres.

 

MANY Earth-orbiting spacecraft have a nadir-pointing requirement that requires full three-axis attitude stabilization. Typical nadir-pointing stabilization systems use passive gravity gradient momentum wheel and magnetic torquers, or fully active systems that include a suite of reaction wheels, control moment gyros, or thrusters to implement full three-axis control. Traditional attitude stabilization systems are impractical for micro and nanosatellites . Reaction wheels and momentum wheels do not fit within its limited weight and power budget. Magnetic torque coils can be used, but it is difficult to guarantee global stability with only magnetic torque because of the inability to torque about the local magnetic field direction.

 

If the design does not require orbital maneuvering capability and, therefore,  they do not include thrusters. At altitudes below 400 km, aerodynamic drag torque tends to overwhelm the gravity gradient torque for practical lightweight deployable boom designs.

 

Sensors

The sensors are the main resource when it comes to determining the attitude of a vehicle or object with precision. The so-called actuators are also necessary to provide the torque that is needed to position the satellite in the desired attitude, with the algorithms that can interpret the data and drive actions in the Spacecraft. The discipline that studies these three essential elements for the control of attitude (sensors, actuators and algorithms) is referred to as GNC, which stands for Guidance, Navigation and Control.

 

The limitations of micro-satellite due to the limits of weight and power and requirements of low design cost and high precision requires reducing unnecessary attitude sensors and controllers to ensure the precision of attitude control.

 

Microsatellites and nanosatellites carry a IMU – Payload: An inertial measurement unit uses accelerometers and gyroscopes to measure the inertial components of a system resulting from motion or vibration. The data can be used to calculate the position and orientation of the vehicle at any later moment and also to study the vibration characteristics of the system. Onboard computer is the Command and data handling part of the satellite. It also schedules and controls the payload operations on the satellite. It acts as the heart and brain of the satellite for its survival in space environment. Current Microsatellites and nanosatellites  are capable of autonomous operation, which implies that they do not require intervention from the ground station for its basic survival. Based on the requirements with respect to the operation of the payloads, the ground station can send commands to the satellite.

 

There are different types of sensors to control attitude in space, such as solar sensors, horizon sensors or star trackers.

 

Actuators

Reaction wheels are the standard AOCS actuators for most earth observation and science missions with high-resolution optical payload. Reaction Wheel assembly (RWL) consists of 5 wheels in a skewed configuration, 4 mainly used and 1 for cold redundancy. The main set of 4 wheels provides actuation during any operating mode with the exclusion of ASM. For the specific application, key parameters are the allowed control torque and the momentum storage capacity.

 

Russian Scientists Create Prototype Of Smart Nanosatellite Control System

Scientists from Russia’s Samara National Research University have built a prototype of an intelligent system to control nanosatellites that is expected to be tested during a real mission next year, Andrey Kramlikh, the project manager and associate professor of the Inter-University Department of Space Research, told Sputnik.

 

“The first experiment we want to conduct using our control navigation and communication system is a project to study the ionosphere of Earth. We want our system to be installed at the nanosatellite, which is now under development by our team, and we hope that in 2021 we will be able to test the system in real flight conditions,” Kramlikh said. The smart system created in Samara will be able to make independent decisions in a variety of emergency situations in orbit. The project has been financed by the Russian Science Foundation. According to the scientists, the system will increase the reliability of the spacecraft and reduce the cost of its electronic components.

 

Solar MEMS Sensors

Horizon Sensors are optical instruments that detect the light at the edge of the Earth’s atmosphere, which means the horizon. This type of technology offers satellites and Spacecraft orientation in relation with the geometric planes of our planet. The horizon sensor we make in our company is the Horizon Sensor for Nano and Micro Satellites (HSNS). It is a Quad Thermopile sensor for Earth detection and Nadir vector determination. This device measures the infrared radiation from Space and from Earth with 4 IR-eyes, providing accurate and reliable detection and attitude determination, writes Solar MEMS.

 

In contrast, Solar Sensors are devices that detect the direction of the Sun. This is one of the special strengths of Solar MEMS and we have various types of sensor to match the requirements of the mission. There are the SSOC and nanoSSOC sensors, that latter being a sensor with two axes that offers high precision at low cost and already there are 500 of them working in nanosatellite.

 

Finally, Star Trackers are optical devices that can measure the positions of the stars by using photosensors or cameras. The newest developments of Solar MEMS include the STNS, a low-cost star tracker based on a CMOS image sensor for highly accurate satellite attitude determination. The device captures images of a Star Field with an internal camera device and identifies star constellations to determine the orientation of the satellite in an absolute reference frame and attitude with high accuracy. In fact, the first Solar MEMS Star Tracker will soon be in orbit on a European Space Agency (ESA) satellite.

 

Building and flying the world’s smallest Spacecraft Attitude Determination and Control System, reported in Dec 2020

Attitude determination and control systems (ADCS) have been studied and implemented in Cubesat size spacecraft for around a decade. The one thing that has to be always kept in mind is the necessity for optimization. Anything that might require too much processing or power has to be either turned on only when absolutely necessary or has to be replaced by more simple and less demanding hardware.

 

The size of the satellite is both ADCS’s best friend and worst enemy. Due to the small size and weight of the spacecraft, the ADCS can embrace the minimalistic dimensions with ease while maintaining enough force for reliable control. The complications arise from the necessity to optimize every single aspect of the satellite. In this case, the biggest limitation arises from the power budget. ADCS can easily become a black hole for the energy stored by the solar panels. For this reason, ADCS is only called for when required. This strategy induces certain uncertainty but it ensures the satisfactory lifetime of the satellite while delivering results when needed.

 

Alba Orbital has developed the world’s smallest ADCS on board the Unicorn-2 platform. This State of the art ADCS is the first ever ADCS to be integrated onto a PocketQube class satellite, developed in collaboration by Dr. Matteo Cerriotti of the University of Glasgow and Alba Orbital’s leading spacecraft engineers. Their work delivered a capable ADCS that powers multiple missions. This system is based on magnetorquers, reaction wheels, sunsensors, light dependent resistor magnetometer and gyroscope all of which enables the satellite to perform exemplary detumbling as well as accurate inertial pointing. But the road does not end there. Alba Orbital is determined to perfect the Unicorn-2 platform to empower a constellation of PocketQube satellites that will deliver accessible, high-quality data sets.

 

Building on Unicorn-2’s flight heritage from 2019, three more Unicorns are manifested to launch on board a SpaceX Falcon 9 as part of the sold out ‘Alba Cluster 3’ mission, which is the largest PocketQube launch cluster in history to date. If you would like to know more about how Unicorn-2 can support your in-orbit mission, feel free to get in touch at contact@albaorbital.com or visit the Unicorn-2 webpage.

 

References and Resources also include:

https://www.solar-mems.com/do-you-know-what-an-attitude-control-system-is/

http://www.albaorbital.com/new-blog/2020/11/24/building-and-flying-the-worlds-smallest-spacecraft-attitude-determination-and-control-system

About Rajesh Uppal

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